The present invention is related to a voltage detector of a battery assembly. More particularly, the present invention is related to a voltage detector of a battery assembly for detecting a voltage across both terminals of each unit cell of the battery assembly in which a plurality of unit cells comprised of batteries are connected to each other.
For the power source of an electric car, a battery assembly is used in which a plurality of unit cells comprised of batteries are connected in series or parallel to each other so that the battery assembly can have a necessary large capacity. As a voltage detector to detect the voltage across both terminals of each cell of the above battery assembly, for example, the resistance type potential dividing voltage detector shown in FIG. 6A is known. Concerning this type voltage detector, for example, refer to JP-A-2000-134818.
In the resistance type potential dividing voltage detector shown in FIG. 6A, switches are successively turned on in the order of switches S1 and S4→switches S2 and S4→switches S3 and S4. Due to the foregoing, both end potions of unit cell V1 are connected to the potential dividing circuit composed of resistors R1 and R4 both end portions of unit cell V2 are connected to the potential dividing circuit composed of resistors R2 and R5, and both end portions of unit cell V3 are connected to the potential dividing circuit composed of resistors R3 and R6 in this order.
According to potential dividing outputs V1′, V2′ and V3′ of these potential dividing circuits, the controller 4 can detect the voltages across both terminals of the units cells V1 to V3. This controller is supplied with electric power from the low voltage power source of 5 V which is different from the battery assembly. Further, this controller is connected to the ground line which is different from the ground line of the battery assembly. In the resistance type potential dividing voltage detector described above, when values of resistors R1 to R6, which are potential dividing resistors, are made to be high, it is possible to substantially insulate the low voltage system controller 4 and the battery assembly from each other.
However, in the above resistance type potential dividing voltage detector, the controller 4 is connected to the battery assembly via switches S1 to S4. Therefore, it is impossible to attain a perfect insulation between the controller 4 and the battery assembly.
Concerning the voltage detector capable of attaining a perfect insulation between the controller and the battery assembly, the flying capacitor type voltage detector shown in FIG. 6B is known. Concerning this voltage detector, refer to JP-A-2002-15212. In the flying capacitor type voltage detector shown in FIG. 6B, switches are successively turned on in the order of switches S7 and S8→switches S8 and S9→switches S9 and S10.
Due to the foregoing, both end portions are connected to capacitor C in the order of unit cell V1→V2→V3. According to the voltage across both terminals of capacitor C which has been electrically charged by unit cells V1 to V3, the controller 4 detects the voltages across both terminals of unit cells V1 to V3. In this connection, when the voltage across both terminals of capacitor C is detected, switches S11 and S12 are turned on, and the voltage across both terminals of capacitor C is outputted into the controller 4. At this time, when the battery assembly and capacitor C are separated from each other by turning off switches S7 to S10, the battery assembly and the controller 47 can be insulated from each other.
However, in the above flying capacitor type voltage detector, switches S7 to S12 are turned on and off by the controller 4 in the low voltage system. Therefore, in order to insulate the battery assembly and the controller 4 from each other, it is necessary to use such a switch as a photo-MOS, which is turned on and off when an optical signal is inputted, for switches S7 to S12. In this case, the manufacturing cost is raised. Therefore, for example, if switches S7 to S12 are controlled by a logic circuit in the high voltage system to which electric power is supplied from the battery assembly, it becomes unnecessary to use the photo-MOS. However, in this structure, switches S7 to S12 can not insulate the battery assembly and the controller 4 from each other.
In the resistance type potential dividing voltage detector shown in FIG. 6A, detection errors may be caused due to the deviation of resistors R1 to R6 which are the potential dividing resistors connected to the unit cells V1 to V3. These resistors R1 to R6 also deviate according to a change in the ambient temperature.
On the other hand, in the flying capacitor type voltage detector shown in FIG. 6B, when a common capacitor C is connected to the unit cells V1 to V3, it is possible to eliminate detection errors which are caused by the deviation of capacitors C connected to each unit cell V1 to V3. However, even in the case of the above capacitor C, its capacity deviates due to the ambient temperature in the same manner as resistors R1 to R6. Due to the foregoing, detection errors may be caused.